The heart is the center of a person's circulatory system. The left portions of the heart draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart draw deoxygenated blood from the body organs and pump it to the lungs where the blood gets oxygenated. Contractions of the myocardium provide these pumping functions. In a normal heart, the sinoatrial node, the heart's natural pacemaker, generates electrical impulses that propagate through an electrical conduction system to various regions of the heart to excite the myocardial tissues of these regions. Coordinated delays in the propagations of the electrical impulses in a normal electrical conduction system cause the various portions of the heart to contract in synchrony, which efficiently pumps the blood. Blocked or abnormal electrical conduction or deteriorated myocardial tissue causes dysynchronous contraction of the heart, resulting in poor hemodynamic performance, including a diminished blood supply to the heart and the rest of the body. Heart failure occurs when the heart fails to pump enough blood to meet the body's metabolic needs.
Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Examples of tachyarrhythmias include supraventricular arrhythmias or supraventricular tachycardias (SVTs), and the more dangerous ventricular tachyarrhythmias which include ventricular tachycardia (VT) and ventricular fibrillation (VF). A supraventricular tachyarrhythmia (SVT) is an arrhythmia that originates from the supraventricular region, such as the atrium, the sinus node, the AV node or AV junction. Examples of SVT include atrial tachyarrhythmia (AT) as well as AV and AV Nodal Reentry Tachyarrhythmias (AVNRT). Atrial tachyarrhythmia includes atrial tachycardias such as atrial flutter, and further includes atrial fibrillation, for example. SVT can be conducted through the AV node, thus resulting in a ventricular tachyarrhythmia associated with the SVT. Thus, an atrial tachycardia can evolve into more serious arrhythmias like ventricular tachycardia.
Some SVTs are chronic in nature, whereas others are not chronic. The duration of these non-chronic SVTs can range from a time period of less than a minute to a time period of several days. An example of a non-chronic SVT is paroxysmal atrial tachycardia (PAT), which also may be referred to as paroxysmal SVT, AVNRT or AV reentry tachycardia. PAT is a type of rapid atrial arrhythmia characterized by brief periods of sudden-onset and often abrupt termination of atrial tachycardia. The sudden onset of the tachycardia is caused by micro-reentry within the AV node or macro-reentry between the AV node and a bypass tract, and can be associated with uncomfortable and annoying symptoms such as lightheadedness, chest pain, palpitations, anxiety, sweating and shortness of breath.
Cardioversion, an electrical shock delivered to the heart synchronously with the QRS complex, and defibrillation, an electrical shock delivered without synchronization to the QRS complex, can be used to terminate most tachyarrhythmias. The electric shock terminates the tachyarrhythmia by simultaneously depolarizing the myocardium and rendering it refractory. A class of cardiac rhythm management (CRM) devices known as an implantable cardioverter defibrillator (ICD) provides this kind of therapy by delivering a shock pulse to the heart when the device detects tachyarrhythmias. Some SVTs, such as PAT, can be difficult to treat because it typically is not considered to be lethal enough to warrant defibrillation shock treatment. Another type of electrical therapy for tachycardia is anti-tachycardia pacing (ATP). Modern ICDs typically have ATP capability, and deliver ATP therapy or a shock pulse when a tachyarrhythmia is detected.
Cardioversion/defibrillation consumes a relatively large amount of stored power from the battery and can cause patient discomfort. It is desirable, therefore, to terminate a tachyarrhythmia whenever possible without using shock therapy. Devices have therefore been programmed to use ATP to treat lower rate tachycardias and to use cardioversion/defibrillation shocks to terminate fibrillation and certain high rate tachycardias.